I have finally rooted my Arc and give a try to the ro.sf.lcd_density change (by the way, there are two identical lines in the file, the one to modify is the first one...). Is the 170 value the only one to use, or can we try different values ? If not, how is this value determined ?...
Any explanation on this would be appreciated for my "personnal culture"...
Thanks a lot !

The code basically tiers various pixel density (screen resolution vs screen size) values. The tier for the ICS-like tablet UI ends at 178 or 179 (I forget at the moment; at 179 or 180 and up, it becomes the phone-like UI we all know and loathe, centering the back/home/multitasking buttons and moving the clock, radio icons, and notifications up to the shade), so 170's just a nice, convenient number to use.